(1) Field of the Invention
The present-invention relates generally to high-speed underwater vehicles. More particularly, this invention relates to stabilization of cavitating flows past high speed underwater vehicles to improve performance.
(1) Description of the Prior Art
Currently, high-speed underwater vehicles can be used in offensive and defensive roles. Often they may be designed to reduce drag by generating enough gas to envelope them in a gas filled cavity. These vehicles have been referred to as supercavitating vehicles, and enveloping them in a gas-filled cavity improves speed and maneuverability, and provides other favorable capabilities. FIG. 1 schematically shows a representative rocket-propelled, supercavitating body, or vehicle 10 capable of operating in the re-entrant jet regime. Supercavitating vehicle 10 has a forebody 12 and midbody 14, and nose portion 16 of forebody 12 has a cavitator 18 that is associated with cavity 20. Cavity 20 starts at the low-pressure point. In the case shown, this is at the salient edges of cavitator 18. Cavitator 18 is sized to generate cavity 20 at the design speed, design depth and design cavity pressure of supercavitating vehicle 10. Consequently, cavity 20 almost completely envelops vehicle 10 to reduce the total drag of vehicle 10 by significant reduction of the drag component attributed to skin friction. For most practical cases of interest, maintenance of the designed cavity pressure of cavity 20 will require ventilation of the cavity by an internal gas source (not shown) that vents gas through ventilation ports 22 in forebody 12 at nearly the same hydrostatic pressure as the ambient fluid, or water 100, at the designed operating depth of vehicle 10.
The configuration of vehicle 10 incorporates nozzle extender, or blast tube 26 that locates exit plane 28 of propulsion nozzle 30 in some optimal location with respect to the aftermost point, or transom 14a of midbody 14. It is important that cavity 20 be maintained large enough to envelope fore and midbodies 12 and 14 of vehicle 10 for satisfactory operation of supercavitating vehicle 10. Entrainment of vented gases by propulsion plume 34 expelled through propulsion nozzle 30 tends to stabilize dimensions of cavity 20 over an expected range of operating conditions. Moreover, impingement of re-entrant jet 102 of ambient water 100 on aftward face, or transom 14a of midbody 14 can result in additional drag reduction beyond that already associated with operation in the supercavitation regime.
The configuration of supercavitating vehicle 10 may be subject to variation. These variations can be in the relative lengths of forebody 12, midbody 14, and nozzle extender 26. Variations can also be made in the transverse dimensions of cavitator disc 18, midbody 14, nozzle extender 26, and propulsion nozzle 30, and the profiles of forebody 12 and propulsion nozzle 30. In addition, nozzle extender 26 could be boat-tailed, and appropriate tailoring can be performed with respect to the configurations and arrangements of the cavitator 18, ventilation ports 22, the dimensions of cavity 20, the operational conditions, and other design dimensions and parameters, as part of the design of vehicle 10. Such variations of these parameters are not significant for the invention to be described below, nor are any variations associated with the shapes of control surfaces, appendages, sensors, and any geometrical features intended to account for gravitational effects on the shape or extent of cavity 20.
Closures of hydrodynamic cavities in flows of liquids for which the freestream velocity has a horizontal component fall generally into one of three categories: (1) oscillating re-entrant jets; (2) toroidal vortex shedding; and, (3) twin-vortex flow systems. The type of closure that is observed for a particular flow depends on the cavitation number (the flow parameter characterizing the tendency to maintain a cavity) and the Froude number (the flow parameter characterizing the relative importance of fluid inertia and gravitational acceleration). Vehicles, such as vehicle 10, that are designed to run in a supercavitating condition for reduced drag are often provided with a gas ventilation system to maintain a cavity of sufficient dimensions to envelope most of the body. The rate of ventilation required to maintain the cavity is dependent on the cavity closure type: high flow rates are required in the twin-vortex regime in which gas easily exits the cavity via the vortex system, moderate flow rates are required in the toroidal vortex shedding regime in which gas is entrained by the ambient liquid in a series of coherent vortices, and relatively low flow rates are required in the oscillating re-entrant jet regime in which gas entrainment by the main flow of liquid is impeded by the complicated interaction between the liquid flowing through the re-entrant jet and the liquid just outside the boundary of the cavity. Even in the oscillating re-entrant jet regime, the required ventilation rate can be significant since the re-entrainment of liquid is associated with a secondary re-entrainment of the ventilation gases.
For many applications, supercavitating vehicles are restricted to operations in the re-entrant regime due to constraints on the cavitation and Froude numbers. However, re-entrant jet flows of liquid are inherently unsteady, because liquid entering the gas cavity via the jet must disturb the cavity boundary to exit the system back into the main flow. Such unsteadiness can cause control problems, can limit the maneuverability of the vehicle, and can be associated with increased self-generated noise levels for any onboard acoustical sensors.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a device to stabilize the cavitating flows past self-propelled high-speed supercavitating vehicles, such as torpedoes and other supercavitating high-speed bodies for improving controllability and maneuverability, reducing gas ventilation rates needed to maintain cavities, and reducing self-generated noise for incorporated acoustical sensors.